This invention relates to mechanical energy conversion devices that include inductor-alternators and methods for providing increased output power, and more particularly toward high-efficiency inductor-alternator energy conversion devices including brushless motor-generators having low inductance armature windings. The inductor-alternators of the present invention include armature windings that are located in a single magnetic air gap of an unusually high reluctance field circuit in place of traditional armature windings that are enclosed in the high permeability parts of a lower reluctance field circuit.
Mechanical energy storage devices have been proposed and/or used for a variety of different purposes. For example, flywheel energy storage devices may be used as a backup source of energy in situations requiring a continuous supply of reserve power in the event of a primary power source failure (i.e., failure by a utility company supply). In such situations, it is often required that a secondary power source supply a nominal amount of power for a certain time period so that various pieces of equipment utilizing primary power may be shut down in a relatively normal fashion, rather than the instantaneous shut down that would occur from a loss of primary power without a backup supply. While flywheel energy storage devices provide several advantages over the use of a bank of chemical batteries (another traditional, short-term, secondary power source), both devices are often combined with an emergency generator that provides long-term secondary power.
One application of such a device is in a paper mill where substantially liquid paper pulp is sprayed onto a rotating wire mesh and then carried through a long series of rollers through ovens to remove the moisture from the pulp. It may take several minutes for the liquid pulp to pass through all of the ovens before the pulp has dried and reached the end of the line where it is rolled up onto high speed spools. An instantaneous loss of power under such circumstances would be catastrophic. Therefore, paper mills must have some form of short-term secondary power to keep all of the equipment running while the pulp supply is shut off and the remainder of the pulp already on the production line is processed.
Chemical batteries suffer from various deficiencies including bulkiness, lack of reliability, limited lifespan, high maintenance costs and relatively low safety. For example, chemical batteries require relatively constant and complex recharging, depending on the type of batteries involved to insure that the batteries continue to operate efficiently and maintain their full storage capacity. Additionally, chemical batteries raise various safety considerations due to the general nature of the large quantities of caustic chemicals involved. Typical large battery installations often require special venting and air-conditioning systems for the dedicated battery storage rooms.
Conventional flywheel energy storage devices have their own set of deficiencies. For example, achieving a high level of energy conversion efficiency is often difficult due to energy losses. Moreover, the energy losses often generate significant heat. In order to minimize the energy losses due to air drag friction, some flywheel system are designed to operate in a vacuum (e.g., see Benedetti et al. U.S. Pat. No. 4,444,444). The vacuum condition demands, however, that heat generation in the rotating components be minimized because rotor heat in a vacuum can only be dissipated by radiation or conduction through bearing surfaces that are small and have limited heat conducting capacity. Moreover, the vacuum condition typically necessitates the use of brushless motor-generators because brushes tend to exhibit extremely short lifespans when operated in vacuum conditions. Brushless motor-generators, however, typically utilize heat generating components such as rotating rectifier assemblies and rotating coils that add further complications.
Another application for these energy storage devices is power generation. Utility companies, for example, have varying demands for power. These variations may be seasonal, daily or a combination of the two. One system that attempts to address peak demand power is Studer et al. U.S. Pat. No. 4,077,678 (hereinafter, "Studer"). Studer shows a flywheel energy storage device that includes a composite, flywheel rotor (having an inner rim of magnetically soft iron) magnetically suspended around a ring-shaped rotor. Permanent magnets generate magnetic flux in the air gaps between the rotor and the stator. During low demand periods, the flywheel is operated as a motor under utility power. As demand increases, the device may be operated as a generator. Studer, however, is inefficient because it requires significant input energy, either in the form of high field coil current or the amount of permanent magnet material used, for normal operation. This inefficiency is substantially due to the use of three air gaps (i.e., axial air gap 36 and radial air gaps 46) in the magnetic circuit. Moreover, Studer has a relatively low energy and power density because: (1) much of the magnetic circuit mass of that device does not rotate; (2) a central rotor core limits the tip speed of the active air-gap (and thus the amount of voltage generated per amp-turn of field coil current); and (3) a large empty space in the air gap does not contribute to power generation.
To provide increased energy density, a solid steel rotor may be employed, such as that shown by Kalsi U.S. Pat. No. 4,159,434 (hereinafter, "Kalsi"). Kalsi's solid steel rotor includes a shaft for rotation that enables the device to be operated at higher speeds than rotors that must be mounted to a shaft (e.g., because the stress concentrations at the shaft bore limit rotational speed). Kalsi also attempts to provide improvements in efficiency by utilizing axial air gaps (see Kalsi FIG. 2, air gaps 31) that reduce the number of air gaps from three to two. Unlike the air gaps of Studer (where the radial air gaps generate no voltage), both air gaps of Kalsi are used to generate voltage. Kalsi, however, suffers from some of the deficiencies previously described. For example, Kalsi has a relatively low energy density because a large portion of the magnetic circuit--the laminated pair of rings--does not rotate. These stationary rings also suffer from high AC field core losses that result in a significant generation of heat. Moreover, Kalsi provides armature windings embedded within iron slots cut into the stator core that increase armature inductance resulting in reduced power density.
Another device characterized by high energy storage is described in Kober U.S. Pat. No. 3,428,840 (hereinafter, "Kober"). Kober describes a pair of iron rotors mounted on a rotating shaft. This configuration is better than Kalsi and Studer in energy density because the entire magnetic circuit mass rotates (only approximately half of the magnetic circuit rotates in Kalsi and Studer). Moreover, like Kalsi, Kober employs two functional axial air gaps in a single magnetic circuit instead of the three air gaps described by Studer. However, the energy density of Kober is somewhat limited by the mounting of the rotors on a shaft and the resultant stress concentrations due to the bore therein that limit the tip speed of the active air gap (resulting in a relatively low quantity of voltage generated per amp-turn of field coil current). Further, Kober utilizes copper field coils mounted to the rotors that limit the rotational speed, and therefore the stored kinetic energy and armature voltage per amp-turn of field coil current, of the device. Moreover, Kober is limited to non-vacuum applications because it uses electrical brushes to power the rotating field coils, which results in high aerodynamic losses.
In view of the foregoing, it is an object of this invention to provide an improved energy conversion device that efficiently provides high output power, including a compact design resulting in a high power density.
It is also an object of the present invention to provide an improved energy conversion device capable of achieving high rotational speeds to store mechanical energy.
It is an additional object of the present invention to provide methods and apparatus for reducing the effects of air gap energy losses on high speed energy storage devices.
It is a still further object of the present invention to provide improved energy conversion devices that may be produced at low costs when compared to currently known technologies.